Could just skip keeping track of it, and just have certian missions\plot tied into the vaule of the stock, that are random, or part of the corp missions..
For instance, BioCorp, gives you 50 stock, to do this mission. you do it, you get the stock.. no change in stock price, since it was a common mission.
Later, BioCorp has a mission that is critical, since if it fails, they don't get that new factory in Town X. So they could email you, or you could find out about it in the news.. and if you take it, and succeed well.. factory is built in Town X, it sets off a trigger the shoots up the price of the stock after said mission, it runs on its own timer.
So after said mission, price of stock is up, since you saved BioCorp money, and they got to build that new factory and you have a certain amount of time before the variable on the stock price is reset, that you can take advantage of and sell the stock for a profit above normal.
Of course, if you fail, or ignore that mission, stock goes down for a while, or it is ignored since you did nothing and price of stock stays with in normal variables.